(i) Field of the Invention
The present invention relates to an electrochemical method and apparatus for treating precious metal milling barren solutions to remove heavy metals which may be present in the barren solution, having leached from the ore or having been added as an integral part of the milling process.
(ii) Description of Prior Art
In many precious metal ore milling operations the precious metals, for example gold and silver, are leached from the comminuted ore by a solution of cyanide in water. During this leaching process heavy metal contaminants can also leach from the ore. The heavy metals form complex cyanides thereby reducing the concentration of free cyanide in solution available for the leaching of the precious metals.
The cyanide leaching solution when containing precious metals is called a pregnant solution and when the precious metals have been removed from it is termed barren. To reduce the costs of milling precious metal ores it is necessary to reuse the barren solution for leaching further ore. Excessive amounts of heavy metal contamination interfere with the metallurgical processes involved in the leaching and subsequent recovery of the precious metals. Therefore the reuse of barren solutions is limited by the presence of heavy metal contaminants. Disposal of the unusable barren solution to the environment is a further cost to the process. Copper is a common heavy metal contaminant in such milling solutions. Removal of copper from barren solutions allows a greater reuse of the barren solution and at the same time frees the complexed cyanide for reuse as a leaching agent for the precious metals. Accordingly it is desirable to remove copper from barren solutions in order, to decrease the cost of destroying cyanide in unreusable barren solution, to decrease the cost of precipitating and disposing of heavy metals in unreuseable barren solution and to decrease the amount of cyanide used in the milling process.
At the present time the problem of copper dissolution is solved either by adding excessive amounts of cyanide to the leaching solutions thereby increasing the cost of processing, and limiting this solution to those ores with a higher gold content, or by acidification of the barren solution to free the cyanide as a gaseous product to be readsorbed in fresh leaching solution.